1. Field of Invention:
This invention relates generally to mass flowmeters, and more particularly to a Coriolis-type meter in which the fluid to be metered is conducted through a helically coiled flow tube that includes a double loop that functions as a tuning fork whose tines are free to vibrate in phase opposition so as to torsionally oscillate as a function of mass flow.
2. Status of Prior Art:
A mass flow rate meter is an instrument for measuring the mass of a fluid flowing through a conduit per unit time. Most meters for this purpose measure a quantity from which the mass can be inferred, rather than measuring mass directly. Thus, one can measure the mass flow rate with a volumetric flowmeter by also taking into account pressure, temperature and other parameters to compute the mass.
A Coriolis-type mass flowmeter provides an output directly proportional to mass flow, thereby obviating the need to measure pressure, temperature, density and other parameters. In this type of meter, there are no obstacles in the path of the flowing fluid, and the accuracy of the instrument is unaffected by erosion, corrosion or scale build-up in the flow sensor.
In the Roth U.S. Pat. No. 3,132,512, a Coriolis-type mass flowmeter is disclosed in which a flow loop vibrating at its resonance frequency is caused to oscillate about a torque axis which varies with fluid flow in the loop. This torsional oscillation is sensed by moving coil transducers.
The Cox et al. U.S. Pat. Nos. 4,127,828 and 4,192,184 show a Coriolis-type meter having two U-shaped flow loops arranged to vibrate like the tines of a tuning fork, torsional oscillation of these loops in accordance with the mass of the fluid passing therethrough being sensed by light detectors. In the Smith U.S. Pat. No. 4,222,338, electromagnetic sensors provide a linear analog signal representing the oscillatory motion of a U-shaped pipe. Electromagnetic sensors are also used in the Smith et al., U.S. Pat. No. 4,492,025, in which the fluid whose mass is to be measured flows serially through two parallel U-shaped pipes which together operate as the tines of a tuning fork.
Because a double-loop Coriolis-type meter functions as a tuning fork, the minimum power required to oscillate the two loops occurs at their natural frequency. When the two loops vibrate as a tuning fork with respect to an anchor at the junction of the two loops, they will alternately draw together to a minimum spacing and then separate to a maximum spacing; hence the angular velocity vector for one loop will always be opposite to the angular velocity vector for the other loop. And because the flow through the two loops is the same, the loops will be subjected to opposing torques by reason of the opposite angular velocity vectors. As a consequence, the two loops are caused alternately to twist toward and away from each other.
A double-loop tuning fork configuration also provides a more stable operation than two single loops in parallel relation, for the mass flow is common to both loops and does not depend on evenly splitting the flow between the two loops. This results in a dynamically balanced pair of loops and a substantially decreased sensitivity to external vibratory forces.
However, because the loops of the tuning fork are anchored at their center which is the junction of the two loops as well as the inlet and outlet ends, such anchoring strongly inhibits deflection of the loops. As a result, velocity sensors of the type used in the prior art are not sufficiently sensitive to provide an adequate signal for mass flow measurement.
To overcome this drawback, the Herzl U.S. Pat. No. 4,747,312 discloses a mass flowmeter of the Coriolis type in which the fluid to be metered is conducted through a pipe which is coiled to form a double loop. The pipe is anchored on a stationary frame at its inlet and outlet ends and also at its center which is the junction of the two loops to define a tuning fork in which the identical loops on either side of the anchored center function as tines that are free to vibrate as well as to twist.
An electromagnetic driver mounted at the vertex of the double loop is electrically energized to cause the loops to vibrate, in phase opposition, at the natural frequency of the tuning fork. The fluid passing through the double loop is subjected to Coriolis forces, thereby causing the vibrating loops to torsionally oscillate in accordance with the mass flow of the fluid. Capacitance sensors are symmetrically mounted on the respective loops to yield signals having a difference in magnitude and phase that depends on the amplitude of the torsional oscillations, these signals being applied to a differential amplifier whose output is proportional to the mass flow of the fluid.
The Herzl double-loop meter exhibits serious defects in meter performance. In the Herzl meter, the double loop is supported on a stationary frame in which the double loop is anchored at three points; namely, at its inlet and outlet end and also at the junction of the two loops forming the double loop. The double loop is therefore highly sensitive to forces from external sources that may give rise to vibration of the frame or produce torsional or bending moments.